RICHMOND, Va.—The baby sister of former Virginia Gov. Bob McDonnell testified in his federal corruption trial on Tuesday that her brother was an honest man who wouldn’t take, or hide, anything from anyone.

“There are two sides to Maureen,” said Mr. McDonnell’s sister, who is also named Maureen McDonnell. “Sometimes you’re not sure which one is going to show up. She can be very sweet and very tender. The other side of Maureen can be manipulative, can be very deceptive. I’ve seen her hide things from Bob, and try to work a situation to her benefit and not to be truthful about that.”

The McDonnells are facing 14 charges, including obstruction and conspiracy, for accepting $160,000 in loans and gifts from Jonnie Williams Sr., the founder of a dietary supplement company who was seeking support for his experimental product. The McDonnells have pleaded not guilty.

Prosecutors rested their case last week, with Mr. Williams as their star witness. Bob McDonnell’s team is presenting his case this week and the 60-year-old former governor is expected to testify any day now. His wife’s defense team will present her case thereafter.

The former governor’s sister, 11 years his junior, was called by Bob McDonnell’s defense team to make a few key points: that the 38-year McDonnell marriage has long been on the rocks, that the former first lady was controlling the relationship with Williams, and that Bob McDonnell had access to his wealthy sister’s money so he was not in the financial straits that prosecutors describe.

Bob McDonnell’s sister got choked up at times describing her sister-in-law, who she said she’d known for 40 years. She said it was the former first lady, “a vitamin nut,” who pushed the relationship with Williams, maker of an anti-inflammatory product made from tobacco called Anatabloc.

“She is passionate, almost to the point of great irritation, about vitamins,” McDonnell’s sister said.

The sister’s testimony made for an awkward day in court. The McDonnells were seated with their lawyers just a few feet away from the witness stand, while their family member talked about their marriage went “from bad to worse” after Bob McDonnell was elected and how Bob McDonnell was trying to “get help” for his wife.

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